Dogs can be trained to sniff out bowel cancer, even when the disease is in its early stages, researchers in Japan claim.

In a series of experiments that involved sniffing the breath or stool samples of patients, a specially-trained labrador retriever proved nearly as good at identifying those with cancer as a conventional colonoscopy examination.

The team, led by Hideto Sonoda at Fukuoka dental college hospital, said some dogs have such a keen sense of smell that they can detect minute traces of chemicals that appear to circulate in the bodies of people who have cancer.

The finding builds on previous experiments in which researchers used dogs to sniff out cancers in the skin, lungs, bladder and ovaries.

Writing in the journal Gut, the researchers describe how the eight-year-old dog was trained to distinguish between the smell of a patient with colorectal cancer and a healthy volunteer, using samples of their breath alone.

In later tests, the dog correctly identified 33 of 36 people with bowel cancer after sniffing their breath, and 37 of 38 cancer cases after sniffing a stool obtained from the patient.

The findings are expected to help scientists identify aromatic chemicals in the body that betray the early signs of bowel cancer and can be detected by medical sensors.

An effective test for bowel cancer, called the faecal occult blood test, picks up hidden blood in a stool sample, but is only able to detect early stage disease in one in 10 cases, the researchers said.

"It would be extremely difficult to use dogs as part of routine testing for cancer, and that's why further research in this area is concentrating on finding out more about the molecules given out by tumours, to see if they could be detected in other ways."